Hydrodynamic overview and seasonal variation of estuaries at the eastern sector of the Amazonian coast

At the eastern sector of the Amazonian coast a macrotide, abundant rainfall and a low relief result in numerous estuaries. According to variation in rainfall, fluvial discharge and geology, there is also a substantial variation among these estuaries. To evaluate this diversity, short measuring campaigns were carried out, including currents, water levels, discharge, salinity, bathymetry and sedimentology. Results indicate four main types of estuary: typical macrotidal estuaries; estuaries with large fluvial discharge; shallow, frictionally-dominated macrotidal estuaries, with negligible fluvial discharge; and estuaries with structural control. At the typical macrotidal type the ebb phase was longer than the flood, however the current velocities tend to be higher during the ebb, due to substantial fluvial discharge influence, especially during the wet season. Regarding salinity, longitudinal gradient and vertical homogeneity is observed. Mean water depth is at the same order of the tidal range. Mean fluvial discharge varies from several tens to few hundreds, within this category. Where the fluvial discharge is negligible, there is no significant longitudinal salinity variation, although the seasonal variation is substantial. There, substantial asymmetry and flood dominance is observed. However, there is no substantial seasonal variation in the current pattern. Mean depths are small, compared to the tidal range. However, some river valleys and estuaries seem to be associated to local faults and present depths of up to ten times the tidal range and relatively weak currents. Results have showed that the differences are substantial among the estuaries, although they are closely located and similar in form.